[glibc/nsz/bug27072] Make libc symbols hidden in static PIE

[Szabolcs Nagy <nsz@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4b939181155fc70ccb708530c1244eb0c2fafbef

commit 4b939181155fc70ccb708530c1244eb0c2fafbef
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Wed Jan 6 11:31:04 2021 +0000

    Make libc symbols hidden in static PIE
    
    Hidden matters with static PIE: extern symbol access in position
    independent code usually involves GOT indirections which needs
    RELATIVE relocs in a static linked PIE. Hidden visibility avoids
    indirections and RELATIVE relocs on targets that can access symbols
    pc-relative.
    
    The check should use IS_IN_LIB instead of IS_IN(libc) since all
    static libraries can use hidden visibility to avoid indirections,
    however the test system links objects from libcrypt.a into dynamic
    linked test binaries so hidden does not work there.  I think mixing
    static and shared libc components in the same binary should not be
    supported usage, but to be safe only use hidden in libc.a.
    
    This is an optimization. But on some targets (i386) it cannot be
    applied because hidden visibility PIE ifunc functions don't work,
    so it is gated by NO_HIDDEN_EXTERN_FUNC_IN_PIE.
    
    From -static-pie linked 'int main(){}' this shaves off 71 relative
    relocs on aarch64 and reduces code size by about 2k.

Diff:
---
 include/libc-symbols.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index ea126ae70c..f4dd735555 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -434,13 +434,18 @@ for linking")
   strong_alias(real, name)
 #endif
 
-#if defined SHARED || defined LIBC_NONSHARED \
-  || (BUILD_PIE_DEFAULT && IS_IN (libc))
+#if defined SHARED || defined LIBC_NONSHARED
 # define attribute_hidden __attribute__ ((visibility ("hidden")))
 #else
 # define attribute_hidden
 #endif
 
+/* Mark all symbols hidden in static PIE libc to avoid GOT indirections.  */
+#if BUILD_PIE_DEFAULT && !defined NO_HIDDEN_EXTERN_FUNC_IN_PIE \
+    && IS_IN (libc) && !defined LIBC_NONSHARED
+# pragma GCC visibility push(hidden)
+#endif
+
 #define attribute_tls_model_ie __attribute__ ((tls_model ("initial-exec")))
 
 #define attribute_relro __attribute__ ((section (".data.rel.ro")))